The invention relates to a method for operating an inverter system and also to an inverter system working according to said method. An inverter system in this case is interpreted as a network-connected inverter together with a network filter connected upstream thereof on the network side.
Active network-connected inverters are typically used for exchange of energy between an AC network and a DC link circuit in each case. They are therefore of great importance both for drive systems, wherein motor inverters use the regulated DC voltage as their input, and also increasingly for energy generation and energy storage, for example with a battery in the DC link circuit.
A characteristic of active network-connected inverters (FIG. 1) is the capability for effective power exchange in both directions, i.e. from the network into the link circuit to supply a drive and also in the opposite direction, in order for example to feed braking energy or stored energy or energy generated with PV modules into the network. In addition capacitive or inductive reactive power can also be output.
As with motor-side inverters, this is done in the prior art with switching semiconductor elements, for example IGBTs in a B6 bridge circuit. Through fast switching between two or more voltage stages an approximately continuous curve of the AC output voltage can thus be created over a switching or pulse period. Depending on the technology and output power used, the switching/pulse frequencies (for reasons of consistency the term switching frequency or switching frequencies is used below under all circumstances) lie between a few 100 Hz and a few 100 kHz, typically they are 2 kHz to 20 kHz.
The AC output voltage of a pulse inverter, in addition to the fundamental wave (network frequency) and low frequency network harmonics, thus also contains components at the switching frequency (with sidebands) and their multiples. As well as the desired fundamental wave, the low-frequency network harmonics can be regulated-out or at least reduced by a regulation of the inverter. The unwanted frequency components on the other hand, that are caused by the switching of the power semiconductors, must be reduced by suitable switching technology means, usually in the form of network filters, far enough for the connection standards relevant in each case at the Point of Common Coupling (PCC) to be adhered to and for other devices in the network not to be disrupted.
The diagram in FIG. 2 shows an example of the frequency curve of such a network filter with reference to the transmission function between inverter output voltage and resulting voltage at the PCC. Above the (actually unwanted) resonance point, a better reduction of the switching-frequency voltage disruptions is achieved with increasing frequency. For the layout of the system as a whole, the aim is to achieve an optimum from conflicting requirements:
On the one hand the inductance values in the filter should be as small as possible, in order to minimize costs and filter size. However the resonance point is shifted by this to higher frequencies and the effect reduces at a specific switching frequency (above the resonance).
On the other hand the switching frequency should be as low as possible, in order to keep the switching losses in the inverter small and the efficiency high. A reduction of the switching frequency however (with the given filter) leads in its turn to a lower filter effect and thus to increased switching-frequency components in the PCC voltage.
The task of the system developers is therefore to find an optimum possible compromise between filter outlay and switching frequency. The point to be considered in this case is that the filter effect is also strongly dependent on the network inductance. If specific values for the switching-frequency interference voltages are to be adhered to, then assumptions must be made in relation to the network inductance to be expected and the system must be designed for a worst-case scenario.
The following equation applies for the relationship between network inductance LN and relative short-circuit voltage uK 
      u    K    =            2      ⁢      π      ⁢                          ⁢              f        Netz            ⁢                          ⁢              L        N            ⁢                          ⁢              S        KS                    U      Netz      2      with the short-circuit power SKS of the network. For the situation shown by way of example in FIG. 2 it is thus approximately true to say in simple terms that the filter for the case with uK=1% must be dimensioned too large by a factor of 5, so that even with uK=10% , a specific limit value for the switching-frequency interference can be adhered to.
If the characteristics of the network are known in advance and are approximately constant, then the filter and/or the system could be adapted in a cost-optimal way. As a rule however, this knowledge is not available and moreover the network parameters can vary greatly over time. Thus the system has to be equipped with attenuation that may not be needed and account has to be taken of corresponding component costs and/or unnecessary switching losses.